Fire alarm system with remote central station

ABSTRACT

To provide unambiguous indication of fire alarm conditions in a central station, in which a plurality of fire alarm sensing lines come together, to which sensors are connected which, in case of fire, have low electrical resistance, the central has a current limiter which limits the current in case an alarm is sensed to a first limited value (Im), the predetermined current being selected with respect to the alarm sensors such that, until the limiting current is reached, the electrical resistance of the sensor is higher than a predetermined value and, when the predetermined current has been reached, the electrical resistance drops below that resistance value; the signal central further having a voltage sensor to sense voltage drop at the connection point of the lines leading to the sensors which, when an excessive voltage drop across a certain line is sensed at the predetermined current value (Im) further causes transmission of a higher current than the predetermined current(Im), at least for a limited period of time, in order to be able to clearly differentiate between alarm conditions, possible line short circuits or interruptions, or non-alarm conditions during line trouble.

United States Patent [191 Herrliberg et al.

[ June 28, 1974 FIRE ALARM SYSTEM WITH REMOTE CENTRAL STATION [75]Inventors: Otto Meier Herrliberg; Andreas Scheidweiler, Stafa, both ofSwitzerland [73] Assignee: Cerberus AG, Mannedorf,

Switzerland 22 Filed: June 28,1973

21 Appl.N0.:374,795

[30] Foreign Application Priority Data July 17, 1972 Switzerland10654/72 [52] U.S. Cl 340/409, 340/412, 340/228 R,

[5]] Int. .Cl. G08b 17/10 [58] Field Of Search 340/412, 409

[56 References Cited UNITED STATES PATENTS 3,665,46l 5/l972 Gnogi340/409 Primary Extzminer-Thornas B. l-labecker Attorney, Agent, orFirm-Flynn & Frishauf [57] ABSTRACT To provide unambiguous indication offire alarm conditions in a central station, in which a plurality of firealarm sensing lines come together, to which sensors are connected which,in case of fire, have low electrical resistance, the central has acurrent limiter which limits the current in case an alarm is sensed to afirst limited value (I,,,), the predetermined current being selectedwith respect to the alarm sensors such that, until the limiting currentis reached, the electrical resistance of the sensor is higher than apredetermined value and, when the predetermined current has beenreached, the electrical resistance drops below that resistance value;the signal central further having a voltage sensor to sense voltage dropat the connection point of the lines leading to the-sensors which, whenan excessive voltage drop across a certain line is sensed at thepredetermined current value (I,,,) further causes transmission of ahigher current than the pre' determined current(l,,,), at least for alimited period of time, in order to be able to clearly differentiatebetween alarm conditions, possible line short circuits or interruptions,0r non-alarm conditions during line trouble.

14 Claims, 7 Drawing Figures I 3 L1 RL.

u l u u E FIRE ALARM SYSTEM WITH REMOTE CENTRAL STATION Cross referenceto related patent and application: US. Pat. 3,767,917, Oct. 23, 1973 andUS. Ser. No. 374,310, filed June 28, 1973, both assigned to the Assigneeof the present application.

The present invention relates to fire alarm systems and moreparticularly to a central fire alarm station which is connected by meansof electrical lines to a plurality of remotely located fire sensors, toprovide an alarm, and preferably an indication which one of the sensorshas responded.

Fire alarm sensors frequently include units which.

normally have a high electrical resistance, and, upon sensing of analarm condition, change the electrical resistance to a very low value,change in resistance on a connected line being sensed in a centralstation. This central station permits constant monitoring of a pluralityof sensing units. The individual sensors or sensing units can belocated, as desired, in the particular locations where fire or excessivetemperatures are to be monitored' The connections are usually over lineswhich may be quite long. These lines are of the low voltage type, forexample similar to telephone lines. Suitable fire alarm sensing unitsare ionization-type fire alarm sensors, optical sensors, smokedetectors, temperature sensors, or various other types of sensors whichreact to ambient conditions indicative of fire, or results'of fire.Frequently a plurality of such fire sensors may be connected, inparallel, through a common line pair, and a number of such pairs areconnected to the signal central. 1

Upon occurrence of a fire, the electrical characteristics of the firesensor change. In one form, an abrupt change in resistance results upontransition from normal condition to alarm condition of the sensingunits. The signal central is provided with units which sense the changein current flowing to the supply lines of the sensing units themselves.By means of a current sensing device, the abrupt change in current canbe sensed, to obtain an alarm indication. The system as describedfunctions well, but is subject to false alarms if there is trouble inthe electrical supply line. Rapid increase in electrical current flowingthrough the line may be indicative of a fire, that is, a change in thesensor resistance; a similar effect, however, occurs upon short circuit,or defects in the supply line. The resistance of a short circuitconnection may be in the order of magnitude of the resistance of a firesensor, when in alarm condition. Thus, current of increased magnitudewill flow through the lines, due to the breakdown in insulationresistance between the supply wires, although no fire exists, butrather, an insulation breakdown, and/or a short circuit.

In fire alarm systems it is equally important not only to signalexisting fires, but also to suppress false alarms. The disadvantage ofthe aforementioned system could be avoided by so constructing theindividual alarm sensors that the resistance of a sensing unit, when inalarm condition, has a value which is intermediate between quiescentresistance and maximum possible short circuit resistance of the line andthe unit. Unfortunately,

there are limits to such resistance adjustment, since the alarmresistance of individual alarm units cannot be changed to be of anyelevated desired value. The alarm resistance of any fire sensing unitmust be clearly differentiated from the overall quiescent condition ofall the units, connected in parallel, including a possible resistanceterminating the supply lines, so that there will be a clear distinctionbetween current flowing through the lines under alarm conditions as wellas under ordinary quiescent monitoring conditions. It is also frequentlycustomary to utilize an incandescent lamp, connected in the alarmsensing unit, to provide an indication of which unit responded; therelativelylow resistance of this lamp may, practically, be the entirealarm resistance of the sensing unit.

The order of magnitude of maximum short circuit current may reach theorder of magnitude of the current flowing to a sensing unit,particularly if the sensing unit is connected to a long line and theshort circuit occurs at the end of the long line. Further, if thesensing unit is connected to a long line, there will be a voltage drop,when the alarm indicator lamp is connected, due to the current flowingin the long signaling line from the sensing unit to thecentral station,so that the brightness of the alarm indicator, when illuminated, willsubstan-' tially depend on the line resistance, and thus on theindividual location of any one of the sensing units.

It is not possible to reduce the maximum possible short circuitresistance, as seen from the central unit, that is the maximum lineresistance, since the costs of increasing wire sizes rise quickly. Inextensive and long fire alarm systems, in which sensing units arelocated at substantial distance from the signal central, the costsincrease rapidly unless standard conductors of low cross sectionalareas, such as telephone cables, can be used. 1

It is an object of the presentinvention to provide a fire alarm system,with a fire alarm central station, in which the foregoing disadvantagesare largely avoided, and to provide a fire alarm system central which iscapable of .clearly distinguishing, without ambiguity, betweenresponseof a fire alarm sensor, and a short circuit in the connectingline between the central and the sensor itself. Further, the entire lineshould be capable of being monitored, up to the last sensing unit in theline, and the apparatus should be so arranged that upon either shortcircuit or interruption in the connecting line (for example, a wirebreak) a trouble indication is obtained,'even if the connecting linesare of considerable lengths.

SUBJECT MATTER OF THE PRESENT INVENTION The central station includes acircuit which limits the current going to a line connected thereto to apredetermined value. The fire alarm units or sensors areso constructedthat the electrical resistance thereof is greater than a firstpredetermined resistance value, if this predetermined current flows.Above this limiting value of predetermined current, electricalresistance of the 'fire alarm unit drops to a second resistance'value,less than the first resistance value thereof. The signal central further includes a device to determine, and evaluate the voltage dropacross the line terminals leading to the fire extinguishers, and,further, a .device to at least temporarily increase the current flowingto the line, if the voltage drop, as sensed in the signal centralexceeds a certain limit when the first limiting current flows.

The central thus, in case there is a change in current flow,automatically initiates a test sequence, limiting the increased current,measuring voltages, and, as a result reaching a logical decision whethera fire alarm sensor has'responded or the line became defective.

of the connection line, the signalling lamp being con n'ected to thisvoltage, therebyavoiding the well known difficulties arising in the useof. incandescent lamps due to the tight interrelationship betweenvoltage and light output, as well as life of illuminating units.

The invention will be described by way of example with reference to theaccompanying drawings, wherein:

FIG. 1 is a simplified highly schematic diagram of a complete fire alarmsystem;

FIG. 2 is a graph illustrating the voltage-current characteristics of asensing element; 2' FIG. 3 is a circuit diagram of a complete sensingelement; v a 3 FIG. 4 is a circuit diagram of another embodiment of asensing'element; I I

' FIG. 5 is acircuit diagram, schematically, of a signal central; v

FIG. 6 is a simplified diagram of a termination unit; and

' A signal central 1 (FIG. 1) is connected over lines L L to a pluralityof fire sensors. In the example shown, a first sensor F isconnectedcomparatively closely to the central; the lines L L continue, andafurther sensor F is connected remotely from the central/The equivalentresistance of the lines is shown by resistors R The signal central 1 hasa voltage supply V which supplies power to the output, to which thevarious sen sors are connected.

Under. ordinary, quiescent conditions, that is, when the sensors are notinfluenced by a fire, the resistance of all sensors is relatively high.The characteristic is shownby a dashed line D,,, FIG. 2. The lines Land- L have only a small quiescent current' flowing therein. If theresults of a fire affect a sensor, then it will switch into an alarmcondition. The resistance of thesensor will jump to a lower value andthe current flowing in the 'lines will substantially increase.The-signal central 1 includes a current limiter 3 which permits thecurrent toris'e to a maximum value l (FIG. 2). A current detector(Ammeter 2) detects this current inthe signal central. Current detector2, on the one hand, controls an alarm and signalling device 5, and alsotriggers an alarm. If necessary, simultaneously or with some delay,

an external alarm device A can be energized. A voltage detector 4is'connected to the terminals of lines L,, L

In accordance with the present inventionfsens'ors with a predeterminedvoltage-current characteristics FIG. 7'shows '5 simplified equivalentconnection diagram ofa sensor. I

level. The slope, in this level'region, is so low that the sensor may beconsidered to have'a saturation voltage U The currentl,,,,'at the kneeof the curve. is the current which is the limiting current, asdetermined by the current limiting device 3, FIG. 1, in the central 1.

v If one were to draw resistance lines, 'of'various slopes, and passingthrough the intersection between the curve D and the current line I,,,,then the behavior of the system, with various line resistance, willbecome clear. A

. comparatively slightly inclined line will be indicative of low lineresistance, that is, R A more steeply inclined line would berepresentative of a much higher line resistance, R The intersection ofthese resistance lines with the ordinate then provides-the division oftheentire voltage U being supplied by the central 1, into the voltagedrop Up across the sensor, the line drop of the lines U and the voltagedrop within the central 1 itself, U As seen in FIG. 2, the voltage Uarising at the terminals of the connected lines L L2 is always greaterthan the saturation voltage U of the sensor,for'all finite lineresistances R in view of the relationship:

- v f UM 0 UL The voltage-current characteristics of a short circuit ina line is entirely different from that of one of the fire sensors inaccordance with the invention. The characline K. As can be seen,theintersection of current I with line K, to which a line resistance of Rcorresponds, will cut the ordinate belowthe saturation voltage U for awide range of resistances. This means that, in case of a shortcircuit,'a measured voltage U will be apparent at the signal central,which is below that of the saturation voltage of the first sensor. Thus,by analyzing the terminal voltage in the central by means of a voltagesensor 4 (FIG. 1) such as a volt meter, it can be determined if theresulting change in the system, that is, current flow and voltagerelationships, are due to a response of a sensor, that is, a real firealarm, or are due to line trouble, for example a short circuit, de-

pending. upon whether the measured voltage U is greater than thesaturation voltage U of the fire sensor, or less. It will be seen thatthe relationship will be true up to resistances in which R tan 7 in FIG.2. The graphs of FIG. 2 likewise show that I,,, should, preferably, beselected to be as-low as possible, so that the system may be useful fora wide range of line resistance values, within which short circuit, andalarm conditions can be clearly and unambiguously distinguished.

For sensing an alarm, first, a comparatively low current I,,, is beingused in order to be able to differentiate from a short circuit withinawide range of line resistances. After an alarm condition, that is, atrue fire alarm, has been sensed, the signal central can increase thecurrent to operate an indicator'lamp, in pulses, to an alarm current I.'In this region, the characteristics are very fiat, so that thedifferential resistance is low.

This enables providing sufficient current to an alarm indicator, forexample an incandescent lamp, since a.

good deal of power is now available to drive the lamp.

Sensing the terminal voltages additionally can be used to localizea'short circuit. The voltage U at the terminals is a measure of the lineresistance to the short circuit. Similarly, the position of a firesensor can be 10- calized, if a plurality of fire sensors are connectedto the signal central by lines of different lengths. The voltage at theterminals of the connecting lines L L provide an indication of the lineresistance to the fire sensor, that is, to the unit which has respondedand hence to the locality of the fire. i

The voltage sensing device 4, as seen in FIG. 1, also controls thesignalling device 5.'By suitable logic, the voltage sensing device 4inhibits generation of an alarm by unit 5, even if the current detector2 indicates excessive current, if the voltage detector, simultaneously,detects a terminal voltage below the threshold or saturation voltage UIt may even be possible to dispense with the current detector as theunit to provide an alarm, if, for example, by suitable choice of thelimiting current I,,, it can be ensured that change in terminal voltagewill arise upon alarm conditions pertaining at any one of the sensors.The voltage detector 4, alone, then controls the alarm signalling unit5, capable of distinguishing between a true fire alarm condition and aline short circuit.

Besides distinguishing between a fire and short circuit, lineinterruption can likewise be detected. As indicated in FIG. 1, aterminating element E is connected at the end of the lines L L to form acomplete sensing loop.

FIG. 3 illustrates a fire sensor, used in the system of the presentinvention, and operating on the ionization principle, and which has thenecessary current-voltage characteristics. Supply lines 10, 11 haveconnected thereacross, in series, an ionization chamber 12 with twoelectrodes and a radioactive substance. Chamber 12 is open to ambientatmosphere. Additionally, an essentially closed ionization chamber 13,which may be termed a reference chamber, is likewise provided, alsohaving two electrodes and a radioactive substance. Upon penetration ofsmoke, fire aerosols, or the like in ionization chamber 12, theresistance thereof changes so that the voltage drop over the openionization chamber 12 increases. This changes the voltage at thejunction point 14 between the two ionization chambers 12, 13. Junctionpoint 14 is connected to the gate or control electrode of a field effecttransistor (FET) 15. The source electrode of FET 15 is connected to avoltage determined by voltage divider 16, 17; the drain electrode isconnected to control an electronic switch, formed of two complementarynpn and pnp transistors l8, 19, connected in a flip-flop circuit. Thecollector resistors are shown at 20, 21, 22, 23. Base capacitors 24, 25are connectedbetween the respective bases and the emitter supply line.

Operation: If the gate voltage of F ET l5 exceeds the threshold voltage,then the flip-flop will change state, causing transistor 18 to becomeconductive so that resistances 22, 23 will have a voltage appearthereacross. A series circuit including an indicator, typically anindicator lamp 26, and a collector-emitter path of a further transistoris connected in parallel to resistors 22, 23. The base voltage oftransistor 27 is held at the constant level by Zener diode 28. Thevoltage between lines 10 and 11 will thus be essentially independent ofthe current in lines 10 and 11.

FIG. 4 illustrates another embodiment of an ionization-type fire sensor.Two ionization chambers 30, 31 are serially connected across lines 32,33. The junction between chambers 30, 31 is connected to the controlgate electrode of an FET'34, the source of which is ,flop. When FET 34changes stage, transistor 40 becomes conductive, current will tlowthrough resistor 42, transistor 40, as well as resistors 38, 39. At apredetermined voltage drop across resistors 38, 39, transistor 43becomes conductive. Transistor 43 has its base connected to the junctionof the resistors 38, 39 and its collector to the other terminal ofresistor 38, that is, the emitter of transistor 40 and resistors 37.Transistor 44 will become conductive as well. The alarm lamp 45 isconnected in the emitter-collector path of transistor 44. The voltageacross resistors 38, 39 is held at a constant level by transistor 43, inthat additional current is bypassed over its collector-emitter'path.

Transistor 43 and resistors 38, 39 form an equivalent circuit for Zenerdiode 28 in FIG. 3, having a certain inherent loss. It could bereplaced, if desired, by a Zener diode with a resistor in parallel. Thisionization sensor, like the one of FIG. 3, has the voltage saturationcharacteristics D illustrated in FIG. 2.

Similar voltage-current characteristics can be obtained with firesensors having entirely different sensitive elements, that is, sensitiveelements which are not ionization chambers. It is only necessary toprovide a flip-flop circuit which is so arranged, that when a circuitflips into alarm condition, an equivalent constant voltage source, forexample a Zener diode, or a similar circuit, such as a suitablycontrolled transistor or the like is connected; other constant voltagesources may likewise be provided, known in the electronics-art.

The use of a constant voltage source has the additional advantage thatparallel connection with an indicator, such as an incandescent lamp,provides voltae which is voltage independent of the supply voltage,thereby ensuring uniformity in indicating quality, independently of lineresistances, or line lengths.

FIG. 5 illustrates the circuit of the switching central 1 (FIG. 1).Lines L L are connected to terminal 62, 63. A voltage divider formed ofresistors 55, 56 is connected across terminals 62, 63, the tap pointthereof being connected to the base of a transistor 53. The emitter oftransistor 53'is connected over Zener diode 57 to a negative supplyline, which, simultaneously, is connected to terminal 63. Under normal,quiescent, condition, that is, when the line is in order and no fire hasbeen sensed, the voltage U across terminals 62, 63 is roughly that ofthe voltage supplied to the supply terminals 0 and 64, that is, supplyvoltage U This supply voltage can be derived from power mains, anauxiliary battery, or the like. The Zener voltage of Zener diode 57 isso selected, that the voltage drop across resistor 56 is sufficient inorder to hold transistor 53 in conductive state, as long as the voltageU across terminal 62, 63 is greaterthan the threshold voltage U of thealarm sensors A signal voltage appears. at the collector of transistor53-over a resistor 58. This signal voltage is applied to an AND-gate 59and to a trouble signalling device 61. Trouble signalling device 61provides a signal when transistor 53 blocks, that is, when the voltageacross terminals 62, 63 dropsbelow the threshold voltage of the alarmsensor. The collector-emitter path of nected between the line terminal62 and the supply terminal 64. The base-emitter path of a furthesttransistor 47 is connected across resistor 52. The collector of resistor47 is connected over a diode 50 with the base of transistor 46.

, Operation: Under ordinary'conditions, transistor 46 is conductive, bybeing biased into conductive'condition over 52. Upon passage of apredetermined current through resistor 52, transistor 47 resistor 54,connected to the base thereof. If a voltage difference arises betweenthe terminal 62 and 64 which is substantial. for example due to responseof a fire sensor, or a short circuit, then the voltage dropsacrossresistor 52, transistor 47 becomes conductive, and a portion of the basecurrent of transistor 46 will be shunted, in order to counteract furtherincrease of the current flowing over transistor 46. As a result, aconstant current circuit is provided so that between terminals 62,64 aconstant current will flow. The transistors 46, 47 thus act as currentlimiters, or constant current sources, in the sense ot' the' device 3 inFIG. 1.

The collector of transistor 47 is connected to a further input of theAND-gate 59. When transistor 47 becomes .conductive,- and simultaneouslythe terminal voltage U is higher than the threshold voltage U of thefire sensor, then both inputs of the AND-gate 59 will be energized, anda signal will be transferred to alarm device 60. The collector emitterpath of a further transistor 48 is connected in parallel to the resistor52. A collector resistor 51 isconnected in the circuit, as shown. Thebaseof transistor 48 can be controlled by a pulse source'schematicallyshown at 76. When an alarm signal is commanded, transistor 48 iscontrolled, in pulses, to become conductive, so that the overallresistance between terminals 62, 64 changes in pulses. Thus, the currentlimiting effect also changes, in pulses, sothat a sensor which providesan alarm will have current supplied to its indicator lamp which likewiseis a pulsed current, that is, the lamp will flash. This provides clearlyvisible indication of response, much more visible and apparent thanmerely uniform illumination, as would be derived from d-c or higherfrequency a-c operation; further, the overall power being supplied isdecreased. I

-'A fire alarm system having a signal central in accordance with FIG. 5,and tire sensors connected thereto, which have suitable voltage-currentcharacteristics permit clear differentiation of a fire alarm from ashort circuit in the line at any desired location of the line. Break ofa line cannot be sensed by this system alone, however.

Line trouble is a fire alarm system with very long lines can be reliablysensed by including in the lines a terminating element E (FIG. 1) whichprovides an interrupted, pulsed loading when a supply voltage on thelines is applied. This permits measurement of the quiescent signalwiththe. same current detectoras for an alarm, by additionallyevaluating the signal in the central with respect to a-c components, orwith respect to pulse components. The alarm sensing itself is delayed orarranged in such a manner that it does not respond to the short pulsesof the terminating element.

An'active terminating element is seenin FIG. 6. A series circuit of aresistor 67, a capacitor 68, a four-layer semiconductive element withtwo control electrodes, and a resistor 70 is connected between lineterminals 65, 66. A resistor 73,and a capacitor 74 and resistor '75 areconnected in parallel to the first series circuit.

The junction between resistor 73 and capacitor 74 is connected to one ofthe control or gate electrodes of the four-layer semi-conductor 69. Thejunction between capacitor 68 and the four-layer semiconductor 69 isconnected to the tap point of a voltage divider formed of resistor 71,72 and connected across terminals 65, 66. Due to the alternate chargingof capacitors 68, 74, semiconductor 69 is opened and closed in pulsedcycles. Y

If lines L and L; from terminating element E to the signal sensor 1 arein connected, operative condition, then pulses derived from theterminating element E, connected over terminals 65, 66, will beconnected to terminals 62, 63 (FIG. 5) of the signal central. They areamplifiedby transistors 46, 47. The pulses are applied over capacitor 78connected to the collector of transistor 47, capacitor 78 connecting thepulses to a pulse detector 77; Pulse detector 77 is so arranged that acontrol signal is applied to trouble indicating means 61 if no pulsesaresensed by pulse detector 77. Thus, the signal central can determineif supply voltage is applied to the terminating element E, which is anindication that the lines are operative up to the last fire sensors.

The invention was described in connection with ionization fire sensors,but it may be used with other types of tire sensors or alarm sensors,such as flame sensors, smoke detectors, or temperature sensors, or othersensing or transducer devices. FIG. 7 illustrates the equivalent circuitof such a detector. An alarm switch S(A) is connected in series with aresistor R,,, the series circuit of switchand resistor R being connectedacross the lines L L The alarm switch S(A) corresponds to the ionizationchambers with a field effect transistor connected thereto, and thecontrolled electronic switching stage, as described in the precedingembodiment. Under ordinary conditions, that is, when no fire is beingsensed, switch S(A) is open. Thus, under ideal conditions, no currentwill flow in lines L L In actual practice, however, it has been foundthat alarm switches also in. open condition have a certain resistancewhich may be very high, however. This inherent resistance is indicatedin FIG. 7 by the parallel resistance R Under alarm conditions, switchS(A) closes, shunting resistance R so that an alarm current can flowbetween lines L, and l through resistor R,,.

In accordance with the invention, a further parallel circuit isconnected to this alarm resistance R,,. The parallel circuit includes acurrent sensitive switch S( I in series with a further resistor R Thiscurrent sensitive switch S(I) closes automatically when the alarmcurrent through alarm resistance R, exceeds a predetermined currentvalue. This further decreases the resistance between lines L and L byincluding a further parallel circuit comprising resistor R with resistorR This is seen in the voltage-current diagram of the alarm detector bythe knee in the voltage-current characteristic, which can be evaluatedas described in connection with FIG. 2. Resistances R R and RN need notbe linear resistors. Rather,'better relationships are usually obtained,that is, better characteristics, when the resistance elements havecertain non-linear characteristics. For example, the parallel resistor Rmay include the parallel circuit of a Zener diode, and an indicatorlamp. This form provides a very flat characteristic.

The alarm system and the Sensor thus provides an indication not only ofa fire, as sensed by a sensor, but is capable of clearly distinguishing,and indicating additionally short circuits in the line, or an opencircuit. It is possible, therefore, to obtain a signal not only by afire, but also by line trouble, whether short circuit or a break in theline. The terminating element is so constructed that a supervisory ormonitoring current flows therethrough. In order to prevent furtherlimiting of the system parameters by distinguishing between yet anothercurrent level, the terminating element is so constructed that itintroduces an a-c component in the lines, for example by providing apulsed load thereon. A quiescent d-c current to supervise the sensinglines is thus not necessary. This permits using alarm current from thesensors which ismuch less than in known systems in which a quiescentcurrent must be sensed, since current levels need not be distinguished,which is difficult with long lines; The voltage sensing at the terminalsis so arranged that a trouble or line fault signal will be sensed whenthe voltage, at the limited current being supplied to the sensor is lessthan the voltage which the sensor requires; and provides an alarm signalwhen the voltage at the'terminals of the central is between the supplyvoltage and the sensor voltage. This alarm signal can be further madereliable and-unambiguous by combining it with a current detector in thecentral.

Various changes and modifications may be made within the inventiveconcept, and the invention is not limited to any specific type of sensorshown, provided it has the characteristics described herein.

We claim:

1. Fire alarm systems having a central sensing station (1; FIG. and atleast one remote sensor (F F FIGS. 3, 4,7)

wherein the sensor has a high electrical resistance under normal,monitoring conditions, and a low electrical resistance under firesensing alarm conditions;

and wherein connectinglines are provided, connecting the central stationto the sensors, and the central station supplied electrical power overthe lines to the sensors and includes an alarm system to indicate alarmconditions said system comprising means in the central station connectedinto the lines limiting the current flowing to the sensors to a firstpredetermined limit value (1,

said sensors have the characteristic that, under alarm conditions, theelectrical resistance thereof is a first value which is greater than apredetermined resistance and when current up to said first predeterminedlimit value (I,,,) flows to the sensor and, the electrical resistance ofthe sensor is at a second value which is below said predeterminedresistance when current above said first predetermined limit (1, flowsto the sensor;

means in said signal central (1) measuring and evaluating the voltageacross the lines leading to the sensors;

and means in said signal central (1) logically controlled by saidvoltage measuring means at least temporarily increasing current flow tosaid lines upon detection of a. a voltage drop across the lines inexcess of a predetermined value (Us) and t b. current flow through thelines approximately at said first limit value (1 2; System according toclaim 1 wherein the sensor has a non-linear voltage-currentcharacteristic which, when the current exceeds a predetermined leveltherethrough, exhibits voltage saturation (FIG. 2; D

3. System according to claim 1 wherein the sensor comprises firedetection transducer means;

and switching means connected to and controlled by said transducer meansand connecting a circuit of low electrical resistance to the connectionlines connecting the sensor to the signal central (1).

4. System according to claim 3 wherein the sensor further comprisesmeans sensing current flow through said circuit;

alarm indicator means in said circuit, said alarm indicator means beingconnected into the circuit by said switching means. 5. System accordingto claim 3 wherein the transducer means comprises an ion chamberaccessible to ambient atmosphere, the resistance of which increases uponpresence of smoke or fire aerosols;

and said switching means comprises a field effect transistor (FET 15,34) having an input circuit connected to and controlled by said ionchamber.

6. System according to claim 5 further comprising an electronic switchcontrolled by said fieldeffect transistor, establishing an alarm circuitof low electrical resistance in said sensor, said field effecttransistor and said controlled switch comprising said switching means.

7. System according to claim 3 further comprising a transistor connectedto sense current flow through said circuit, and indicating meansconnected in the collector-emitter path of said transistor, to indicatealarm conditions, the transistor being connected to be controlled tobecome conductive when the current in said circuit of low electricalresistance exeeds a predetermined current value. I

8. System according to claim 7 wherein the indicator comprises a lamp.

9. System according to claim 2 wherein the sensor includes a Zener diodeto provide said voltage saturation characteristics, independent ofcurrent flow.

10. System according to claim 1 wherein said voltage measuring andevaluating means comprises means responsive to two or more voltage.levels across the lines leading to the sensors.

11, System according to claim 10 wherein the voltage measuring andevaluation means comprises means sensing a voltage level less than apredetermined threshold level;

and said means logically controlled by said voltage measuring meansincludes the line trouble indicating means and a fire alarm indicatingmeans, the

line trouble indicating means being activated when:

a. the terminal voltage is less than said threshold level and b. thecurrent flowing through said lines is at least at, or above said firstlimit current value (l,,,);

and the alarm means is activated when:

c. the voltage across the terminals is above said threshold level and d.the current flowing through the lines is approximately at said firstlimit value (1,

. 1 1 12. System according to claim 1 1 wherein the voltage level of thethreshold level is approximately the same voltage as the saturationvoltage of the current-voltage characteristic of the sensor.

13. System according to claim 1 further comprising pulse means (76; FIG.5) connected to said central station, said pulse means being energizedupon detection of an alarm condition comprising detection of a voltagedrop across the lines in excess of a predetermined value s);

said pulse source being connectible to said lines and,

' in pulsed cycles, increasing the current through said lines to providepulsed power to the sensor which changed to alarm condition to permitflashing, intermittent illumination of a sensor indicator said lines.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,82 1734 Dated June 28 1974 '.[nventor(H) Q tto MEIER Q nd Andreas SCRPTDWEILER- It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

The first inventor's name should read as follows Otto MEIER Signed andsealed this 12th day of November 1974.

(SEAL) Attest McCOY M. GIBSON JR. Y C. MARSHALL DANN Attesting OfficerCommissioner of Patents F ORM PO-IOSO (10-69) USCOMM'DC 60376-P59 9 U.54 GOVERNMENT PRINTING OFFICE i969 0-386-33L

1. Fire alarm systems having a central sensing station (1; FIG. 5) andat least one remote sensor (F1, F2; FIGS. 3, 4, 7) wherein the sensorhas a high electrical resistance under normal, monitoring conditions,and a low electrical resistance under fire sensing alarm conditions; andwherein connecting lines are provided, connecting the central station tothe sensors, and the central station supplied electrical power over thelines to the sensors and includes an alarm system to indicate alarmconditions said system comprising means in the central station connectedinto the lines limiting the current flowing to the sensors to a firstpredetermined limit value (Im); said sensors have the characteristicthat, under alarm conditions, the electrical resistance thereof is afirst value which is greater than a predetermined resistance and whencurrent up to said first predetermined limit value (Im) flows to thesensor and, the electrical resistance of the sensor is at a second valuewhich is below said predetermined resistance when current above saidfirst predetermined limit (Im) flows to the sensor; means in said signalcentral (1) measuring and evaluating the voltage across the linesleading to the sensors; and means in said signal central (1) logicallycontrolled by said voltage measuring means at least temporarilyincreasing current flow to said lines upon detection of a. a voltagedrop across the lines in excess of a predetermined value (US) and b.current flow through the lines approximately at said first limit value(Im).
 2. System according to claim 1 wherein the sensor has a non-linearvoltage-current characteristic which, when the current exceeds apredetermined level therethrough, exhibits voltage saturation (FIG. 2;Da).
 3. System according to claim 1 wherein the sensor comprises firedetection transducer means; and switching means connected to andcontrolled by said transducer means and connecting a circuit of lowelectrical resistance to the connection lines connecting the sensor tothe signal central (1).
 4. System according to claim 3 wherein thesensor further comprises means sensing current flow through saidcircuit; alarm indicator means in said circuit, said alarm indicatormeans being connected into the circuit by said switching means. 5.System according to claim 3 wherein the transducer means comprises anion chamber accessible to ambient atmosphere, the resistance of whichincreases upon presence of smoke or fire aerosols; and said switchingmeans comprises a field effect transistor (FET 15, 34) having an inputcircuit connected to and controlled by said ion chamber.
 6. Systemaccording to claim 5 further comprising an electronic switch controlledby said field effect transistor, establishing an alarm circuit of lowelectrical resistance in said sensor, said field effect transistor andsaid controlled switch comprising said switching means.
 7. Systemaccording to claim 3 further comprising a transistor connected to sensecurrent flow through said circuit, and indicating means connected in thecollector-emitter path of said transistor, to indicate alarm conditions,the transistor being connected to be controlled to become conductivewhen the current in said circuit of low electrical resistance exeeds apredetermined current value.
 8. System according to claim 7 wherein theindicator comprises a lamp.
 9. System according to claim 2 wherein thesensor includes a ZenEr diode to provide said voltage saturationcharacteristics, independent of current flow.
 10. System according toclaim 1 wherein said voltage measuring and evaluating means comprisesmeans responsive to two or more voltage levels across the lines leadingto the sensors.
 11. System according to claim 10 wherein the voltagemeasuring and evaluation means comprises means sensing a voltage levelless than a predetermined threshold level; and said means logicallycontrolled by said voltage measuring means includes the line troubleindicating means and a fire alarm indicating means, the line troubleindicating means being activated when: a. the terminal voltage is lessthan said threshold level and b. the current flowing through said linesis at least at, or above said first limit current value (Im); and thealarm means is activated when: c. the voltage across the terminals isabove said threshold level and d. the current flowing through the linesis approximately at said first limit value (Im).
 12. System according toclaim 11 wherein the voltage level of the threshold level isapproximately the same voltage as the saturation voltage of thecurrent-voltage characteristic of the sensor.
 13. System according toclaim 1 further comprising pulse means (76; FIG. 5) connected to saidcentral station, said pulse means being energized upon detection of analarm condition comprising detection of a voltage drop across the linesin excess of a predetermined value (US); said pulse source beingconnectible to said lines and, in pulsed cycles, increasing the currentthrough said lines to provide pulsed power to the sensor which changedto alarm condition to permit flashing, intermittent illumination of asensor indicator which is in alarm condition.
 14. System according toclaim 1 including a terminating element (FIG. 1: E; FIG. 6), saidterminating element including variable resistance means cyclicallychanging between higher and lower resistance values, to provide for flowof an a-c component through said lines; means in said signal central (1)responsive to current flow of said a-c component; and trouble indicatormeans in said signal central (1) responsive to cessation of said a-ccomponent to indicate an open circuit or break of continuity of saidlines.